The present invention relates generally to the field of optical output devices, and more specifically to a method and apparatus for accurately controlling the optical energy delivered to a photoreceptive element in a raster output scanning system.
Although applicable to a wide variety of optical output devices, the present invention finds particular utility in Raster Output Scanning (ROS) apparatus. Therefore, the following details and descriptions begin with a background of the present invention in terms of ROS apparatus. ROS has become the predominant method for imparting modulated light information onto the photoreceptor in printing apparatus used, for example, in digital printing, and has found some application in other image forming operations such as writing to a display, to photographic film, etc. Consider, for illustration purposes, what is perhaps the most common application of ROS, digital printing. As is known, the scanning aspect thereof is conventionally carried out by a moving reflective surface, which is typically a multifaceted polygon with one or more facets being mirrors. The polygon is rotated about an axis while an intensity-modulated light beam, typically laser light, is brought to bear on the rotating polygon at a predetermined angle. The light beam is reflected by a facet and thereafter focussed to a "spot" on a photosensitive recording medium. The rotation of the polygon causes the spot to scan linearly across the photosensitive medium in a fast scan (i.e., line scan) direction. Meanwhile, the photosensitive medium is advanced relatively more slowly than the rate of the fast scan in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. (Although, for the purposes of example, this discussion is in terms of ROS apparatus, as will become apparent from the following discussion, there exists many other scanning and non-scanning system embodiments of the present invention. However, as a convention, the word "scan" when referring to fast and slow scan directions will be used with the understanding that actual scanning of the spot is not absolutely required.) The light beam is intensity-modulated in accordance with a serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as sheet paper.
Images to be transferred to the photoreceptor are generally in the form of sampled data in each of the fast and slow scan directions. The sampled data represents the discrete elements, or pixels, comprising the image. Each pixel of the image is reproduced by one or more spots, each of which is formed by exposure of the photoreceptor with one or more pulses of optical energy. Each pulse of optical energy is formed by modulating the intensity of the scanned light beam. For the purposes of example, the word "pulse" when referring to formation of a single spot will be used with the understanding that the "pulse" may be composed of multiple subpulses, whose total energy content is used to form an individual spot.
The size and shape of each exposed spot depends on the optical energy contained in the pulse as well as on the size and shape of the imaged spot. This dependence on energy delivered per pixel results from the existence of a threshold amount of energy that must be delivered to the photosensitive surface before a spot is exposed. The amount of energy delivered to the photosensitive surface is equal to the time-integrated output of the optical pulse. Thus the size of each spot depends on the duration of the optical pulse as well as on its amplitude. In systems currently known to those skilled in the art, a predetermined amount of energy is delivered to the photosensitive surface by turning on the optical beam to a desired amplitude for a fixed time interval. Since the amount of energy delivered to the photosensitive surface controls, inter alia, the spot size, spot profile, etc., variations in the size of the exposed spot may be obtained by varying the amplitude of the optical beam while maintaining a constant pulsewidth or by varying the pulsewidth while maintaining a constant amplitude.
In order to expose reproducible spots on the photoreceptor, the optical energy delivered in each pulse must be accurately controlled. Accurate control is especially important when printing with different gray levels formed by varying the number of exposed spots within a half tone cell or when exposing very closely spaced spots in order to control the formation of an edge. It has been shown that variations in the optical energy as small as 1% of the total energy used to form the spot may be perceived in a half tone or continuous tone image. This implies a need for a high degree of accuracy and reproducibility in the optical energy delivered to each spot, especially in such applications as color and gray-scale printers.
Variations in the optical energy per spot can arise from fluctuations in either the amplitude or pulsewidth of the optical power emitted by the laser source. Amplitude fluctuations in the laser power can arise from many sources, including for example ambient thermal fluctuations, fluctuations in the drive current, and/or the preceding pattern of modulation. Fluctuations in pulsewidth can arise from many sources, including for example the driving electronics, trigger signals, the turn-on or turn-off times of the laser, and/or the preceding pattern of modulation.
Thermal fluctuations are especially deleterious to maintaining constant optical energy per pulse. For example, heating of the laser chip unavoidably occurs when the applied laser current is abruptly increased at the beginning of a pulse. Since a laser's output power generally decreases as temperature increases, this time-dependent, or transient, heating normally causes the power output to decrease or "droop" during the pulse. Furthermore, transient heating during a sequence of pulses can have a cumulative effect on the temperature that depends on the number and frequency of the pulses. For example, if the time between successive pulses is large, the device will be given sufficient time to cool, so that the application of the driving current has a large temperature effect, i.e. droop, during the next pulse. The shorter the time between pulses, the less time the device has to cool between one pulse and the next, leading to a sustained increase in the temperature of the laser. This sustained temperature increase results in a further decrease in amplitude of the output pulse obtained with a fixed amplitude of the input current, leading to a further variation in the energy of each optical pulse.
Additional deleterious effects on the optical energy per pulse occur in a multiple beam ROS employing a monolithic optical source containing more than one diode laser, for example as described in U.S. Pat. No. 4,445,125, which is incorporated herein by reference thereto. In such monolithic sources composed of closely spaced diode lasers, the modulation of one laser induces a variation of power emitted by other lasers, either through electrical, optical, or thermal coupling. The coupling between neighboring lasers on the same chip interacts with the self-heating of each laser produced by its modulated current to produce erratic and unpredictable fluctuations in the output power of each laser. Similar effects may also occur with nonmonoltihic lasers mounted in close proximity such that heat generated in one laser can be coupled into other lasers.
Since the above fluctuations in the power output of single and multiple laser sources occur within each pulse and are very difficult to predict, control, or eliminate, they are commonly not compensated for. However, such fluctuations are undesirable in a raster scanning optical system since they produce variations in the optical energy per spot that become undesirable artifacts on the printed page. Accordingly, there is presently a need in the art for apparatus and methods which provide accurate and reproducible control of the optical energy delivered to each spot of a raster scanning system. These and other problems are addressed by various aspects of the present invention, which will be summarized and then described in detail below.